The ampere per meter (symbolized A/m) is the International Unit of magnetic field strength. It is derived from basic standard units, but is expressed directly in base units and cannot be further reduced.
Consider the interior of a long, cylindrical coil with a single winding and an air core. Suppose that the linear current density in this coil is 1 ampere per meter of displacement as measured along the coil axis. (This expression differs from current density per unit area, which is expressed in amperes per meter squared .) Then the magnetic field strength in the interior of the coil is defined as 1 A/m.
For a given coil, the magnetic field strength is directly proportional to the linear current density. Thus, if the linear current density doubles, so does the magnetic field strength; if the linear current density becomes 1/10 as great, the magnetic field strength also diminishes by a factor of 10.
Sometimes, magnetic field strength is expressed in units called oersteds (symbolized Oe). The oersted is a larger unit than the ampere per meter. Approximate conversions are:
1 Oe = 79.578 A/m
1 A/m = 0.012566 Oe